During summer months, dissolved oxygen (DO) concentrations in the lower Hudson estuary were usually lower than atmospheric equilibrium values. Distributions of DO along the axis of the estuary can be described by three general characteristics. Firstly, surface and bottom values lie on a single trend when plotted against salinity. Secondly, maximum DO concentrations were observed 50-75 km upstream of Manhattan at salinities of 5-15 ppt. Thirdly, the lowest DO concentrations were observed near Manhattan, New York, at salinities of 15-25 ppt and vary systematically with freshwater discharge rate. Minimum DO concentrations during times of similar freshwater discharge were substantially lower for water samples collected between 1978 and 1984 than for those collected between 1989 and 1993. These two periods were also distinguished by occurrences of intense phytoplankton blooms that produced supersaturated DO concentrations at salinities of 5-15 ppt for the period of 1989 to 1993. The increase in the minimum DO concentration in the water near Manhattan is probably in response to improved wastewater treatment at Passaic Valley, N.J.; North River, N.Y.; and other wastewater-treatment facilities.
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